Conventionally, there has been widely known a mask blank comprising a halftone phase shift film (hereinafter referred to as a phase shift film) made of MoSiN, MoSiON, or the like. In the manufacture of such a mask blank, it is usual to form a phase shift film on a main surface of a transparent substrate using a single-wafer sputtering apparatus. In a normal single-wafer sputtering apparatus, a rotary stage on which a transparent substrate is to be placed is provided in the lower part of a film forming chamber and a target is disposed directly above the rotary stage. However, in the case where the normal single-wafer sputtering apparatus is used in the formation of the halftone phase shift film, there has been a problem that the thickness of the film on the outer peripheral side of the main surface of the transparent substrate tends to be relatively small compared to that on the center side thereof due to a shape of the main surface of the transparent substrate being rectangular. The phase shift film is required to simultaneously achieve a function of transmitting exposure light at a predetermined transmittance and a function of producing a predetermined phase difference between the exposure light transmitted therethrough and exposure light transmitted in air for a distance equal to the thickness of the phase shift film. If there is non-uniformity in the thickness distribution in the plane of the formed phase shift film, there is a possibility of the occurrence of variation in transmittance distribution in the plane or the occurrence of variation in phase difference distribution in the plane. When a phase shift film of a material containing oxygen or nitrogen is formed on a transparent substrate by DC sputtering using as a target material a material containing silicon such as MoSiN or MoSiON, since a nitride of silicon or an oxide of silicon has low conductivity, particles due to charge-up tend to be produced on a target surface. There is a possibility that these particles fall onto the transparent substrate located directly below the target surface and enter the phase shift film, thereby forming defects. That is, there is also a problem that the defect occurrence ratio increases.
In order to solve the unique problems that arise when such a rectangular mask blank film is formed by sputtering, use is made of a single-wafer sputtering apparatus disclosed in JP-A-2002-090978 (Patent Literature 1). In this sputtering apparatus, a target is disposed obliquely above a rotary stage, on which a transparent substrate is to be placed, so that both horizontal and vertical distances are ensured between the transparent substrate and the target (see FIG. 4). By forming a phase shift film on the transparent substrate using the sputtering apparatus of such a structure (sputtering apparatus of a so-called oblique-incidence sputtering type), it is possible to prevent the thickness of the film on the center side of the substrate from becoming relatively large and further to reduce defects due to charge-up of a target surface.
On the other hand, a thin film made of a material such as MoSiN or MoSiON tends to have a relatively large compressive stress. In the process of manufacturing a phase shift mask, a transfer pattern is formed by etching a phase shift film. When the phase shift film is partially removed in the formation of the transfer pattern, a region where the phase shift film is removed is released from the compressive stress so that the interval between patterns (space width) increases. In order to minimize this influence, the film stress of the phase shift film should be reduced. In view of this, for example, as disclosed in JP-A-2002-162726 (Patent Literature 2), a heat treatment is applied at a temperature of 400° C. or more to a glass substrate formed with a phase shift film, thereby reducing the compressive stress. With the use of a hot plate, it is difficult to carry out the heat treatment at the temperature of 400° C. or more. In terms of processing a plurality of films at a time, it is effective to use a vertical electric furnace disclosed in JP-A-2002-162726 (Patent Literature 2).
In JP-A-2001-210604 (Patent Literature 3), an optical heating apparatus using a light source in which a plurality of cylindrical incandescent lamps are disposed in a lattice pattern is proposed as an apparatus for heating a glass substrate, a semiconductor wafer, or the like. This optical heating apparatus is featured in that it can uniformly irradiate light onto a surface to be treated and thus can uniformly apply optical heating to the surface to be treated.